In recent years, as an audio signal encoding method having high compression efficiency, parametric stereo coding method has been developed (for example, refer to Japanese National Publication of International Patent Application No. 2007-524124). For example, the parametric stereo coding method extracts space information which represents a spread or localization of sound and encodes the extracted space information. The parametric stereo coding method is employed in, for example, High-Efficiency Advanced Audio Coding version.2 (HE-AAC ver.2) of Moving Picture Experts Group phase 4 (MPEG-4).
In the HE-AAC ver.2, a stereo signal to be encoded is time-frequency transformed, and a frequency signal obtained by the time-frequency transform is down mixed, so that a frequency signal corresponding to monaural sound is calculated. The frequency signal corresponding to monaural sound is encoded by an Advanced Audio Coding (AAC) method and a Spectral Band Replication (SBR) coding method. On the other hand, similarity or intensity difference between left and right frequency signals is calculated as space information, and the similarity and the intensity difference are respectively quantized to be encoded. In this way, in the HE-AAC ver.2, the monaural signal calculated from a stereo signal and the space information having a relatively small data amount are encoded, and thus high compression efficiency of a stereo signal can be obtained.